This invention relates to a low molecular weight, anisotropic pitch consisting of essentially 100 percent mesophase which can be easily and continuously spun to produce essentially completely anisotropic fibers, which fibers can be further processed to produce carbon and graphite fibers having a high Young's modulus of elasticity and high tensile strength.
As a result of the rapidly expanding growth of the aircraft, space and missile industries in recent years, a need was created for materials exhibiting a unique and extraordinary combination of physical properties. Thus, materials characterized by high strength and stiffness, and at the same time of light weight, were required for use in such applications as the fabrication of aircraft structures, re-entry vehicles, and space vehicles, as well as in the preparation of marine deep-submergence pressure vessels and like structures. Existing technology was incapable of supplying such materials and the search to satisfy this need centered about the fabrication of composite articles.
One of the most promising materials suggested for use in composite form was high strength, high modulus carbon textiles, which were introduced into the market place at the very time this rapid growth in the aircraft, space and missile industries was occurring. Such textiles have been incorporated into both plastic and metal matrices to produce composites having extraordinary high-strength- and high-modulus-to-weight ratios and other exceptional properties. However, the high cost of producing the high-strength, high-modulus carbon textiles employed in such composites has been a major deterrent to their widespread use, in spite of the remarkable properties exhibited by such composites.
One recently proposed method of producing high-modulus, high-strength carbon fibers at low cost is described in U.S. Pat. No. 4,005,183, entitled "High Modulus, High Strength Carbon Fibers Produced From Mesophase Pitch". Such method comprises first spinning a carbonaceous fiber from a carbonaceous pitch having a liquid crystal or mesophase content of from about 40 percent by weight to about 90 percent by weight, then thermosetting the fiber so produced by heating the fiber in an oxygen-containing atmosphere for a time sufficient to render it infusible, and finally carbonizing the thermoset fiber by heating in an inert atmosphere to a temperature sufficiently elevated to remove hydrogen and other volatiles and produce a substantially all-carbon fiber. As indicated in said patent, the highly oriented, optically anisotropic, liquid crystal material produced by heating a carbonaceous pitch at a temperature above about 350.degree. C. has been given the term "mesophase", and pitches containing such material are known as "mesophase pitches". As further indicated in said patent, pitches having a mesophase content of more than about 90 percent are generally not employed in the process described herein because the characteristically high molecular weight of such pitches imparts softening temperatures and viscosities to them which are so high as to make them unsuitable for spinning. Thus, such pitches can only be softened to suitable spinning viscosities at temperatures in excess of 400.degree. C., at which temperatures they continue to polymerize to form still higher molecular weight products, rendering the spinning operation impractical.
An improved process for producing carbon fibers from mesophase pitch is disclosed in U.S. Pat. Nos. 3,976,729 and 4,017,327. According to said patents, fibers are produced from a mesophase pitch wherein the mesophase content of the pitch has been formed while agitating the pitch so as to produce a homogeneous emulsion of the immiscible mesophase and non-mesophase portions of the pitch. Mesophase pitches prepared in this manner have been found to possess a lesser amount of high molecular weight molecules in the mesophase portion of the pitch and a lesser amount of low molecular weight molecules in the non-mesophase portion of the pitch, and thus a smaller differential between the average molecular weights of the mesophase and non-mesophase portions of the pitch, then mesophase pitches having the same mesophase content which have been prepared in the absence of such agitation. Although such pitches are more suitable for spinning into fibers than pitches produced in the absence of agitation, they are still two phase systems which, in their liquid state, consist of two immiscible liquids, one the optically anisotropic, oriented mesophase, and the other the lower viscosity, isotropic, non-mesophase. While the presence of the isotropic phase was heretofore considered necessary to plasticize the higher viscosity mesophase and impart thereto flow properties conducive to the spinning of small diameter fibers, the tendency of these two phase pitches to segregate into their component phases continued to hamper spinning operations.
As further disclosed in U.S. Pat. Nos. 3,974,264 and 4,026,788, pitch of a given mesophase content can be produced in substantially shorter periods of time than previously possible, at a given temperature, by passing an inert gas through the pitch during formation of the mesophase at a rate of at least 0.5 scfh. per pound of pitch. In addition to shortening the time required to produce a pitch of a given mesophase content by lessening the amount of low molecular weight molecules in the non-mesophase portion of the pitch which do not form mesophase, or do not form mesophase at a rapid rate, this procedure results in a more easily spinnable pitch by removing those molecules which adversely affect the rheology and spinnability of the pitch. Again, however, the pitches produced in this manner are two phase systems which have less than optimum spinning characteristics because of their tendency to segregate into their component phases.